Magnetic field sensors, such as magnetoresistors, are useful for a variety of applications, such as position sensors when used with a magnet. These magnetic field sensors are typically made with active elements composed of a semiconductor material or semimetal, such as InSb, InAs, In.sub.1-x Ga.sub.x As, GaAs, Si or Bi.sub.1-x Sb.sub.x, along with associated metal contacts, insulating passivation, etc. These devices are designed to give optimum sensitivity, when a magnetic field is applied thereto, to changes in impedance, thermal stability, etc. One form of a magnetoresistor, as shown in U.S. Pat. No. 4,926,154 (J. P. Heremans et al.), issued May 15, 1990, and entitled "Indium Arsenide Magnetoresistor," comprises a layer of a high mobility semiconductor material on an insulating substrate. Metal contacts are on the layer at the ends thereof and conductive shorting bars are on the layer and spaced therealong between the contacts. The shorting bars divide the semiconductor layer into a plurality of small active regions having a relatively small length-to-width ratio and which are all connected in series. The semiconductor layer may extend along a meandering path to increase device resistance.
To use such a magnetoresistor as a position measuring system, it is mounted on a magnet and used along with a ferromagnetic part, such as a movable gear tooth. The movement of the gear tooth across the magnetoresistor changes the magnetic field strength through the magnetoresistor and thereby changes its resistance. One such system is shown in U.S. Pat. No. 4,939,456 (D. T. Morelli et al.), issued Jul. 3, 1990 and entitled "Position Sensor Including a Thin Film lndium Arsenide Magnetoresistor on a Permanent Magnet." When the magnetoresistor is mounted on a magnet made of a conductive material, a dielectric insulating layer is provided between the magnet and the magnetoresistor to insulate one from the other. Also, it has been found desirable to coat the magnet with a high permeability ferromagnetic layer to increase the sensitivity of the device, such as disclosed in U.S. Pat. No. 4,926,122 to T. Schroeder et al., issued May 15, 1990. However, it would be desirable to further increase the sensitivity of the device by concentrating the magnetic field primarily to the active areas of the sensor.